Exhaust systems for high-performance internal combustion engines of the type used on racing cars have been the subject of considerable empirical design work and some theoretical studies. Nevertheless, exhaust systems are often treated somewhat as stepchildren by racing teams in the effort to increase engine performance. They generally are regarded as necessary evils which can contribute only relatively insignificantly to increase, and more conventionally are thought to decrease, engine horsepower.
To the extent that theoretical-based attempts have been made to enhance engine performance through optimization of exhaust system design, most of this effort has been directed toward what is called "ram tuning" of the exhaust header pipes. Ram tuning is based upon the concept that both the intake and exhaust in internal combustion engines take the form of compressed gas volumes, masses or pulses. The exhaust pulses are controlled by opening and closing of the exhaust valve, and if the length of the exhaust pipe is properly selected, tuned, a negative pressure wave can be timed to be present at the exhaust valve when it opens so as to aid or assist the exhaust of consumed gases from the cylinder. Ram tuning of intakes and exhausts is discussed at length, for example, in Cycle World, "Intake and Exhaust Ram Tuning" by Gordon H. Jennings (1962), and will not be repeated in this application.
While ram tuning of exhaust header pipes is possible and advantageous, as a practical matter it is extremely difficult to obtain significant horsepower improvement using this technique. Ram tuning horsepower increases occur only at very precise, and often unpredictable, engine speeds. Moreover, using ram tuning in a four cycle racing engine, you may be able to achieve a horsepower increase of, for example, five horsepower at 6000 rpm in a 450 horsepower engine. At 5900 or 6100 rpm, however, the ram tuning horsepower increase is zero. Nevertheless, the "conventional wisdom" is to attempt to select header pipe lengths so as to achieve ram tuning and horsepower enhancement at some desired engine speed, depending upon the operating and racing parameters.
One of the problems of trying to predict the speed at which ram tuning will occur is that the speed of sound varies with the temperature of the media in which it travels to a significant degree. Additionally, exhaust pipes are not filled with a homogeneous density or pressure of gas. Thus, ram tuning calculations usually are based upon a plurality of assumptions or approximations which seldom, if ever, correlate with the reality of conditions inside an exhaust header pipe. Precise measurement of the exhaust temperature as it exits the engine is seldom available, and this temperature changes dramatically down the length of the pipe. Exhaust system designers, therefore, often theoretically predict pipe length, install the same and then proceed to try to adjust the pipe length until some effect on engine horsepower can be observed.
Another disadvantage in connection with ram tuning is that the length of header pipes must be precise and matched to have any significant effect. The matching and precision in length can pose serious design problems when coupled with the requirements for the racing car chassis. Thus, ram tuning can produce header pipe designs which are not easily accommodated by racing car chassis. The result can be to disregard ram tuning of the exhaust system or to compromise chassis design.
Another approach to exhaust system design has been to employ header assemblies which have collectors that first allow exhaust gas expansion and then converge the gases to increase velocity and attempt to affect engine horsepower. Typical of these header assembly systems are the so called "tri-Y" header collector and the "clover leaf" header collector. FIGS. 1 and 1A of the accompanying drawing show a tri-Y header collector mounted on two header pipes, and FIGS. 2 and 2A illustrate a clover leaf collector mounted on four header pipes.
In FIGS. 1 and 1A, a pair of header pipes 21 and 22 are connected at one end to the exhaust ports of two engine cylinders (not shown). At the other end, a tri-Y collector, generally designated 23, is welded at 24 in a bead around each of the header pipes. Collector 23 is swaged at 26 to substantially conform to the pipes, with a welding at 25 filling the arcuate interstices between the collector and the pipes. Collector 23 has a frusto-conical section 27 which extends to a cylindrical section 28. Mounted on the end of cylindrical section 28 is a further exhaust pipe (not shown), which can be coupled to another header pipe or to a muffler. As will be seen FIG. 1, the swaged indentation 26 extends into frusto-conical portion 27 of collector 23.
It has been found that tri-Y header collectors can be effective in some cases to increase engine horsepower. Collector design, however, largely has focused on the relative sizes of the areas of pipes 21 and 22 and the area of cylindrical collector portion 28 (and to some degree the rate of taper of frusto-conical portion 27). Usually, the area of cylindrical portion 28 of a tri-Y collector will be at least 30 percent less than the combined areas of pipes 21 and 22.
What has been discovered in connection with tri-Y header designs, however, is that increases in lower rpm horsepower can be achieved, but they occur at the expense of a reduction in the high rpm horsepower. Moreover, the increases which can be obtained at the lower rpm are often only 10 horsepower, while at the same time 40 horsepower will be lost at high rpm. Still further, it often is not possible to predict where a tri-Y header will increase horsepower, making it necessary to design collectors empirically for each racing engine.
It will be appreciated that compressed exhaust gas volumes or masses will alternate or be out-of-phase in their discharge from the ends of pipes 21 and 22 by a spacing determined by opening of the respective exhaust valves on the cylinders to which the header pipes are coupled. As engine speed, rpm, increases the spacing between exhaust volumes in the same pipe and in alternate pipes decreases. Thus, the velocity increase produced by tapered collector portion 27 is effective to increase horsepower at low rpm because the exhaust volumes spacing has not become critical. As the engine speed increases, however, the thirty-plus reduction in the combined area produced by tapered section 27 begins to act as a restriction or chock and engine top-end horsepower is decreased.
In the clover leaf header collector of FIGS. 2 and 2A, four primary header pipes 31, 32, 33 and 34 are joined by a clover leaf collector, generally designated 36. Pipes 31-34 are held in side-by-side relation and a diamond shaped end plate 37 is welded to 38 to fill the space between the pipes. Collector housing 36 is swaged at 39 to conform to the exterior of the pipes and then is welded at 41 all the way around the pipes. The collector similarly is frusto-conical at 42 and tapers to a cylindrical section 43 having an area at least 30 percent less than the combined areas of pipes 31-34. As also will be seen from FIG. 1, swaged indentations 39 extend along tapered section 43, and in fact extend into the tapered sections on four sides of collector 36.
Again, however, the primary mechanism for horsepower increase in clover leaf collectors is the discharge of gases into an expansion funnel 43, which thereafter contacts them to increase velocity. Because of the large volume of cloverleaf collector, the horsepower increase does not occur until the top-end of engine speed and there is a bottom end loss. Thus, one might obtain 5 to 10 horsepower increase at high rpms and lose as much as 50 horsepower at low rpms.
With six cylinder engines header collectors can be used to collect exhaust gases from a triangular array of three header pipes. The function and effect is substantially the same as above described in connection with clover leaf and tri-Y collectors.
A third largely empirical exhaust system design tool is the "balance pipe". In any exhaust pipe the sound components will tend to periodically reinforce and cancel each other at locations along the exhaust pipe determined by sound frequencies, pipe temperatures and pipe configuration. These areas of sound reinforcement, heat is generated and the exhaust pipe acts as though it has a restriction in it. This effect can be overcome by expansion regions or by a balance pipe which couples one of a pair of exhaust pipes to the other of the pair at the position along the pipes at which sound reinforcement is occurring.
Balance pipes generally do increase engine horsepower, and also they may remove the sound-induced restriction which would be present in the exhaust system if the balance pipe were not added. The addition of a balance, however, essentially recaptures horsepower loss that would otherwise occur. Again, however, the location of balance pipes is largely empirically determined, for example, by thermal sensing of hot spots along a pipe (sometimes as much as 200.degree. F. hotter).
The net result of considering these various exhaust system header design criteria, however, has been that "conventional wisdom" usually leads the designer to limited horsepower improvements for limited engine speed ranges, largely through empirical testing. Internal combustion engine exhaust system design, therefore, has been somewhat unpredictable and often only of marginal impact on engine performance.
Accordingly, it is an object of the present invention to provide an exhaust system header assembly for an internal combustion engine which is designed in a manner which measurably increases engine horsepower at substantially all normal engine operating speeds.
Another object of the present invention is to provide a header assembly and method for enhancing engine horsepower which can be predictably employed to significantly enhance engine horsepower at specific engine speeds.
Still another object of the present invention is to provide a header assembly and method of enhancing engine horsepower which produces horsepower increases substantially in excess of the horsepower increases which can be achieved through ram tuning.
Still a further object of the present invention is to provide an internal combustion engine header assembly and method for increasing engine horsepower which is not sensitive to the length of header pipes and thereby provides increased flexibility of header and chassis design.
Still another object of the present invention is to provide a header assembly and method which can be used in combination with low-pressure generating mufflers to greatly enhance engine horsepower.
Another object of the present invention is to provide an internal combustion engine header assembly which is easy and inexpensive to construct, durable, can be retrofit onto virtually any engine, is suitable for use with engines of any number of cylinders, and requires less empirical adjustment to achieve significant horsepower increases.
The header assembly and method of the present invention have other objects and features of advantage which will become apparent from, or are set forth in more detail in, the accompanying drawing and following description of the best mode carrying out the invention.